1. Field of the Invention
The present invention relates to film carrier tapes for mounting electronic parts less suffering protrusion of solder resist at the edge of the solder resist layer. The invention also relates to screen masks for producing such film carrier tapes.
2. Description of the Related Art
For mounting electronic parts such as integrated circuits (IC), film carrier tapes for mounting electronic parts having a wiring pattern on an insulating film are employed. The film carrier tapes for mounting electronic parts are generally produced by a process comprising bonding a metal foil such as a copper foil to a surface of an insulating film such as a polyimide film, coating a surface of the metal foil with a photoresist, exposing and developing the photoresist to form a desired pattern of the photoresist, selectively etching the metal foil using the pattern as a masking material to form a wiring pattern composed of the etched metal foil, and then forming a solder resist layer on the wiring pattern except the terminal portions.
In the above-mentioned production of the film carrier tape for mounting electronic parts, the solder resist is applied as follows. A screen mask for solder resist coating in which a screen is stretched on a frame and the screen surface is masked except the area to be coated with the solder resist is used. This screen mask is superimposed on the wiring pattern formed on the insulating film, then the solder resist is fed onto the screen mask, and applied by moving a squeegee to squeeze the solder resist down to the prescribed area of the wiring pattern.
By applying the solder resist as described above, the wiring pattern except the connecting terminal portions can be protected.
In the conventional film carrier tapes for mounting electronic parts, the density of formation of the bonding pads is not so high, so that, as shown in 50-a of FIG. 7, the solder resist layer 50 is often in a simple shape that is mainly made up of straight lines, such as a square or a rectangle. However, with the recent requirements of high integration, the density of formation of the bonding pads 51 has been increased, e.g., as shown in FIG. 5. Therefore, it becomes necessary to form the solder resist layer 50 along the shapes (indicated by auxiliary broken line 60) of the bonding pads 51. For example, the corner portion of the solder resist layer came to be frequently formed obliquely, as shown in 50-b of FIG. 7, or the corner portion of the solder resist layer came to be frequently formed in a shape of a circular arc, as shown in 50-c of FIG. 7. In such a film carrier tape for mounting electronic parts, the bonding pads (connecting terminals) 51 are formed to arrange along the edge 62 of the solder resist layer 50, as shown in FIG. 5. The solder resist layer 50 of such a shape is often formed by moving a squeegee using a screen mask. The direction of the squeegee moved in this case is indicated by an arrow S in FIG. 5 and FIG. 7.
However, when the circular arc-shaped or oblique corner portion of the solder resist layer is formed by slantwise contacting the squeegee with the edge of the screen mask, the excess solder resist on the screen mask comes round to the lower side of the screen mask by the moving of the squeegee. Therefore, the solder resist at the edge portion that is not right-angled or not parallel to the moving direction of the squeegee, e.g., circular arc-shaped edge portion or oblique edge portion, sometimes protrudes from the coating area of the screen mask. That is to say, the applied solder resist comes round to the lower side of the curved portion or the oblique portion of the screen mask, and hence, the solder resist which has come round to the lower side of the screen and protruded to the edge of the masking zone sometimes forms a horn-like protrusion 52 at the edge of the solder resist layer, as shown in FIG. 6. If the horn-like protrusion 52 reaches the bonding pad 51 which must not be coated with the solder resist, a problem of contact failure of the bonding pad 51 occurs.
The present invention overcomes these problems by providing a film carrier tape for mounting electronic parts having a low rate of occurrence of solder resist coating failure at the edge of the solder resist coating layer.
The present invention further provides a screen mask for solder resist coating, which is used for forming a solder resist layer on the film carrier tape for mounting electronic parts and by the use of which the solder resist hardly comes round to the back surface of the screen even when a squeegee is moved.